tire size questions?
 

i'm looking at new wheels and tires and have a few questions? what does the mm offset do? and what backspace should a person look with a 17 by 8 front and 18by 9 or 10 rear? truck is a 70 short with a 31/2 front drop and 5 or 6 inch drop in rear. and should a person go with a 55series sidewall or a 50 series to fill the wheel openings?